1. Field of the Invention
The present invention relates to wireless signals and, more particularly, to a method for mitigating interference of wireless signals through polarization-based processing of sub-bands of a received signal.
2. Description of Related Art
Wireless communications technology has grown immensely in recent years and proliferated throughout the world to enable high speed voice, video, and/or data communication without the need for a physical connection between the sender and receiver. Such devices are often built to operate over a pre-determined band of frequencies, usually designated by some national licensing authority, such as the United States Federal Communication Commission. By using different frequency channels, multiple radios have been able to operate simultaneously without interfering with one another by filtering to eliminate energy in frequencies outside the desired channel. This concept is as old as the invention of the radio. It can often happen, however, that interfering energy (either intentionally or otherwise) impacts the receiver's antenna(s) at frequencies within the desired channel. Oftentimes, such energy is designated as co-channel interference (CCI).
Traditional frequency-domain filtering is useless in mitigating CCI, so other dimensions of processing to deal with the problem have been researched, such as spatial filtering and polarization filtering. In contrast with the frequency-domain filtering, these other types of filtering require the use of multiple antennas (i.e., an antenna array) and some method of combining the signals from each antenna to achieve some mitigation of the interference.
For example, one exemplary description of polarization-based filtering is disclosed in R. Compton, “On the Performance of a Polarization Sensitive Adaptive Array”, IEEE Transactions on Antennas and Propagation, Vol. AP-29, No. 5, September 1981 (herein “Compton”). Compton describes spatial filtering, where an antenna array is composed of single antennas separated by some distance in space from one another. Such an array is limited in its ability to effectively differentiate between signals that arrive from the same direction (i.e., the angle of arrival). Further, Compton discloses how an array composed of orthogonally polarized elements may effectively mitigate one or the other of two signals even when they arrive from the same direction by filtering based on the polarizations of the two signals.
For instance, an electromagnetic wave is composed of both an electric field (E-field) and a magnetic field (H-field), collectively these fields form a right angle. The polarization of the wave may be defined as the direction of the E-field as a function of space and time. Notably, a linearly polarized wave is present if the E-field points in a single direction as it oscillates at a certain frequency. If the E-field traverses a path as time progresses, it may trace an ellipse in general, which is referred to as an elliptically polarized wave. By using an antenna array with two orthogonally polarized antennas (e.g., horizontal and vertical), it is possible to transmit and/or receive many polarizations in azimuth by appropriately combining the two antenna outputs (to receive) or appropriately splitting the signal to be transmitted to feed the two antennas. Such an antenna array is designated as a “dual-polarized antenna” and considers the effect of combining the two outputs of such an array as a means of polarization-based filtering.
Before Compton, most analytical studies of adaptive arrays assumed the presence of co-polarized elements. This assumption, although useful for certain purposes, tacitly eliminated consideration of the effects of signal polarization on array performance. In essence, the assumption was that all signals arrive at the array with the same polarization. If an array receives and uses more than one polarization, its performance can be far superior to one that does not. For example, an array of co-polarized elements predominantly yields poor performance if interference arrives too close to the desired signal. When an array adapts to polarization, however, this difficulty occurs only if both signals have the same polarization as well as angle of arrival. When two signals arrive from the same direction, it is possible to null one signal and not the other, assuming their polarizations are different.
Compton describes an array of two pairs of crossed dipoles and computing the output signal-to-interference-plus-noise ratio (SINR) from this array when a desired signal and an interference signal arrive with arbitrary polarizations and angles of arrival. The result in most cases showed that interference has little effect on the array output SINR, unless it arrives from the same direction and has the same polarization as the desired signal.
In addition, as described in T. Pratt and S. Nguyen, “Polarization Mode Dispersion Characterization of Wireless Channels with Multipath”, submitted to the IEEE Transactions on Wireless Communications, October 2006 (see also U.S. Ser. Nos. 60/887,207 and 60/887,221, both filed 30 Jan. 2007) (herein “Pratt”), the phenomena of polarization mode dispersion (PMD) and polarization-dependent loss (PDL) have been theoretically predicted and empirically observed in typical wireless communications channels. PMD and PDL are phenomena that have been observed and rigorously studied in single-mode optical fiber (SMOF) applications and result in the corruption of a purely polarized signal by dispersion (or spreading) of the signal's polarization as a function of frequency and signal attenuation as a function of the original polarization. The same formalism developed to study global PMD and PDL effects in optical fiber can be directly applied to the wireless channel medium.
Experiments demonstrating the existence of PMD and PDL in wireless channels are disclosed in Pratt. By means of a wireless transmission and the reception of the signal by a dual-polarized antenna, the PMD was observed by breaking the signal into sub-bands and plotting the polarization on the Poincaré sphere as a function of frequency. The Poincaré sphere may be considered a graphical tool that can represent many polarization states. Some useful features of the Poincaré sphere include: a point plotted on the equator of the sphere represents a linear polarization; the two poles represent left- and right-hand circular polarization; and two orthogonal polarizations are represented by points on opposite sides of the sphere. The plot of those polarizations as a function of frequency reveals the frequency-dependent nature of the polarization of the signal as it passes through the wireless medium.
Even when PMD and PDL is taken into consideration, the receiver of a wireless signal receives a significant amount of interference that has been commonly ignored. Mitigating this interference is necessary.
Accordingly, there is a need for a method of mitigating interference of a received signal in wireless communications. It is to such a method and system that the present invention is primarily directed.